1. Field of the Invention
This invention relates to a choke, such as a drilling or production choke.
2. Description of the Prior Art
Chokes are devices commonly used in the oilfields to restrict or shut off pressurized flow through a flowline.
The fluid streams with which chokes are used are frequently laden with abrasive material, such as rock cuttings and sand. These streams can erode or wear through even the hardest steels. Over the years, therefore, there has been much thought and innovation applied to chokes, with a view to improving their durability.
Following below is a short discussion of three types of prior art chokes which are in commercial use today. Some of their strengths and weaknesses, which are of interest, are touched upon.
The oldest of the three types may be referred to as a needle-and-seat choke. A simplified schematic representation of this type of prior art choke is shown in FIG. 1A. The choke includes a hollow body assembly a, which body assembly defines inlet and outlet bores b, c joined by a main bore d. A tubular throttling member or ring e is positioned in the main bore d at the entry to the outlet bore c. A plug or needle member f is mounted on a valve stem, for movement into or out of the throttling ring e. Entry of the needle member f into the throttling ring e forms a restricted annular throttling passage g, defined by the throttling surfaces h, i of the throttling ring and needle member.
It will be noted that the throttling surfaces h, i are large in surface area, they are contiguous to the restricted or high velocity flow zone, and they are relied on to provide the final or complete shut-off. Because of their proximity to the high velocity flow, these throttling surfaces erode rapidly and soon lose their capability for providing an effective shut-off. Also, the annular fluid jet issuing from the annular passage g is found to have a particular capability for eroding the downstream surface j of the outlet bore c. Although abrasion-resistant steel collars are used to form the outlet bore c, wear at this point is still a problem with needle-and-seat type chokes.
The second type of choke may be referred to as the rotating-disc choke. It is shown schematically in FIG. 1B. This choke comprises two abutting discs m, n. The discs are mounted to extend across the main bore of the choke. Each disc defines a semi-circular opening or port o, which is spaced outwardly from the disc's centre point. The upstream disc m is connected with a stem p. This stem may be rotated to bring the port of the upstream disc m into partial or complete register with the port of the downstream disc n. When the ports are in register, they form a throttling passage.
By utilizing a throttling passage which is a port, as distinct from an annular opening, the rotating-disc choke provides throttling surfaces which are reduced in area when compared with those of the needle-and-seat choke. The durability of the rotating-disc choke is significantly improved by this change.
However, significant erosion does occur at the disc surfaces r, s which are disposed transversely to the oncoming high velocity flow. This erosion soon leads to loss of complete shut-off capability. Also, the stream issuing from the partly registering ports is angularly directed. This angularity is induced by the misaligned positioning of the partly registering ports. The angularly directed flow causes damaging erosion of the bore surface downstream of the discs.
The third, and most recently developed, type of choke may be referred to as the advancing-cylinder choke. A recent form of this choke is illustrated schematically in FIG. 1C. An older version is shown in U.S. Pat. No. 4,132,386. With reference to FIG. 1C, the choke comprises a generally tubular nozzle member u, which is slidably disposed in the main bore of the choke body assembly. The nozzle member bore v communicates with the outlet bore w of the choke. The nozzle member bore v is also connected with the choke inlet bore x by aligned ports y, which extend through the sidewall of the nozzle member and provide a throttling passage. A tubular throttling ring member z is also disposed in the main bore of the body assembly. This throttling ring member z may be advanced by a stem to slide over the nozzle member u and throttle the flow through the ports y. When the throttling ring member is approaching the end of its travel to the left, it contacts the nozzle member and biases it to the left. A deformable seal k, positioned between the end of the nozzle member and the body assembly, is thereby outwardly extruded and combines with the throttling ring to provide the liquid-tight shut-off or seal.
The advancing-cylinder choke is improved in that the aligned, opposed positioning of the throttling ports y results in the incoming streams impinging against one another; it is found that downstream wear is thereby significantly reduced. In addition, the utilization of throttling openings which are circular holes reduces wear of the throttling surfaces, as compared with that encountered with the needle-and-seat choke.
However, the advancing-cylinder choke has some undesirable features as well. Erosion of the leading corner 100 of the throttling ring member occurs, due to the proximity of this corner to the high velocity flow. When the surface of this eroded corner is required to cooperate with the deformable seal k to provide the liquid-tight shut-off, failure occurs. In addition, the need for a movable nozzle member requires that a retaining support means be provided, which is expensive.
With the foregoing comments in mind, there is therefore a need for an improved choke which adopts desirable features from the prior art and melds them with new additional features to provide a simple and durable device.